Coating compositions based on hydroxyl-containing polyacrylate resins and crosslinking agents are known, for example, from JP-A 4-1254. In that document, the hydroxyl-containing polyacrylate resins which are employed as binder are obtainable from hydroxyl-containing monomers, alkyl acrylates, alkyl methacrylates, optionally styrene and also optionally ethylenically unsaturated polymers. In that context it is essential to the invention that the polyacrylate resin has been prepared using 4-tert-butylcyclohexyl acrylate and/or 4-tert-butylcyclohexyl methacrylate as monomer component. The hydroxyl-containing monomer employed is, in particular, hydroxyethyl acrylate and/or hydroxyethyl methacrylate.
These coating compositions known from JP-A 4-1254, especially when used as a transparent topcoat over a basecoat film, have the particular disadvantage that, when hydroxyethyl acrylate and/or hydroxyethyl methacrylate are used as OH monomer, the resulting coatings are of inadequate adhesion to the basecoat film. A further disadvantage of these coating compositions known from JP-A 4-1254 is the inadequate pot life of the coating compositions.
EP-B-0 158 161 discloses hydroxyl-containing acrylate copolymers formed by copolymerization of acrylate monomers having at least two olefinically unsaturated double bonds, hydroxyl-containing monomers and further olefinically unsaturated monomers, which, together with melamine-formaldehyde resin as crosslinking agent, can be employed as coating compositions.
It is likewise a feature of these coating compositions known from EP-B-0 158 161 that, especially when used as transparent topcoat over a basecoat film, they have the disadvantage that, when hydroxyethyl (meth)acrylate is used as hydroxyl-containing monomer, they result in coatings with inadequate adhesion to the basecoat film and that the pot life of the coating compositions is inadequate.
EP-A-0 638 591 describes hydroxyl-containing copolymers which can be prepared by bulk polymerization in which one component is included in the initial charge at the beginning of polymerization, and in the subsequent course of polymerization at least two olefinically unsaturated monomers are added, at least one of which contains at least one carboxyl group and at least one is sterically hindered. Also described is the use of such copolymers in high-solids coating compositions. Again, the adhesion of such coating compositions according to EP-A-0 638 591 to basecoat films is inadequate.
DE-A-44 07 415 discloses coating compositions which provide an outstanding solution to the problems resulting from the prior art to date. Such coating compositions include (A) at least one hydroxyl-containing polyacrylate resin obtainable from (a) a cycloaliphatic ester of (meth)acrylic acid, (b) a hydroxyl-containing ester of (meth)acrylic acid, (c) optionally a hydroxyl-containing, ethylenically unsaturated monomer different from (a) and (b), (d) an aliphatic ester of (meth)acrylic acid different from (a), (b) and (c), (e) optionally a vinyl-aromatic hydrocarbon different from (a), (b), (c) and (d), and (f) optionally a further ethylenically unsaturated monomer different from (a), (b), (c), (d) and (e), and (B) at least one crosslinking agent.
When used as transparent topcoats over a pigmented basecoat film, the coating compositions according to DE-A-44 07 415 exhibit markedly improved adhesion to the basecoat film coupled with good chemical resistance and good weathering resistance. The coating compositions can be cured fully at low temperatures and thus can be used for the sector of automotive refinishing, Even when the coating compositions are cured at these low temperatures, the coating compositions rapidly attain full cure while at the same time retaining processability for a sufficiently long period. Moreover, the coating compositions of DE-A-44 07 415 have the advantage of a good topcoat holdout, exhibit very good flow under the conditions of automotive refinishing, have good application properties, and, when used as clearcoat over a basecoat film, cause only little dissolution of the latter.
Recent market requirements necessitate, however, a degree of adhesion which is improved again relative to the coating compositions of DE-A-44 07 415, when the coating compositions are employed as clearcoat for the coating of basecoat films, and increased gasoline resistance.
Problem and solution
The problem of the present invention was therefore to provide coating compositions which, when used as transparent topcoat over a pigmented basecoat film, lead to coatings having an adhesion to the basecoat which is improved relative to the coating compositions known from DE-A-44 07 415 and which have improved gasoline resistance relative to the coating compositions of DE-A-44 07 415. At the same time, the resulting coatings should be of good chemical resistance and good weathering resistance. In addition, the coating compositions should be suitable for automotive refinishing, i.e. they should be able to be cured fully at low temperatures of in general below 120 degrees C, preferably below 80 degrees C. Furthermore, the coating compositions should also attain rapid full cure (rapid dust dryness and freedom from tack and also rapid through-drying) at these low temperatures, while at the same time they should be processable for as long as possible (pot life). Moreover, the coating compositions should exhibit good topcoat holdout and, when used as clearcoat over a basecoat film, should cause only minimal dissolution of the underlying basecoat film and should exhibit a good light/dark effect when viewed at different angles. Finally, the coating compositions should show very good flow and should exhibit good application characteristics under the conditions of automotive refinishing.
This problem is surprisingly solved by a coating composition of the type specified at the outset which is characterized in that component (A) is a hydroxyl-containing polyacrylate resin which is obtainable by polymerizing preferably:
to give a polyacrylate resin (A) having an OH number of from 60 to 180 mg of KOH/g, the sum of the proportions by weight of components (a) to (g) always being 100% by weight and the monomers or monomer mixtures employed as component (b) only being those which, on polymerization of the respective monomer alone, produce a polyacrylate resin and/or polymethacrylate resin having a glass transition temperature of from -10 to +6 degrees C or from +60 to 80 degrees C.
The present invention additionally relates to a process for producing a multilayer coating on a substrate surface using these coating compositions and to the use of the coating compositions in the sector of automotive refinishing.
It is surprising and was not foreseeable that the novel coating compositions, when used as transparent topcoat over a pigmented basecoat film, exhibit markedly improved adhesion to the basecoat film while at the same time having excellent gasoline resistance, good chemical resistance and good weathering resistance. A further advantage is that the coating compositions can be cured fully at low temperatures and thus can be employed for the sector of automotive refinishing. Even when the coating compositions are cured at these low temperatures the coating compositions rapidly attain full cure while at the same time retaining processability for a long period. Moreover, the novel coating compositions have the advantage of good topcoat holdout and, under the conditions of automotive refinishing, exhibit very good flow and good application properties. Finally, when used as clearcoat over a basecoat film, the novel coating compositions have the advantage that they cause only minimal dissolution of the basecoat film and exert only a very slight influence over the metallic effect.